A fixing unit of known type comprises a base element that supports the antenna, provided with a contact surface that is arranged so that it rests on the outside of the vehicle's roof.
A fixing element is also provided, which is connected to the base element through a screw and comprises a plurality of projecting bodies mainly developed according to the connection direction defined by the screw, with the ends slightly parted.
In order to fix the antenna to the roof, the base element and the fixing element are first arranged on opposite sides of the roof at the level of an apposite through hole.
Successively, the screw is screwed so that the ends of the projecting bodies come into contact with the roof.
The above mentioned projecting bodies are elastically deformable so that their ends can be mutually moved near each other, and the fixing element can be snap-fitted into the hole made in the vehicle's roof.
This makes it possible to install the antenna on the roof with the fixing element already previously connected to the base element, so as to facilitate the operation.
However, the known fixing body described above poses the drawback that the fixing element alone does not allow the antenna to be effectively fixed to the roof.
In fact, the deformability of the projecting bodies, which allows them to be moved near each other and then parted, limits the tightening force that can be exerted on the roof by the projecting bodies.
A further drawback due to said deformability lies in that the projecting bodies tend to part excessively during tightening, thus further limiting the tightening force.
Consequently, in the fixing units of known type there is usually a cage suited to contain the fixing element and to limit the deformation of the projecting bodies.
However, the above mentioned containment cage poses the drawback that it increases the number of components of the fixing unit and therefore its cost.
Furthermore, the larger number of components of the fixing unit means an increase in the corresponding assembly time.
A further drawback of the fixing unit described above lies in that the elastic reaction following the deformation of the projecting bodies tends to cause the progressive loosening of the screw, especially in the presence of vibrations which are inevitable in a vehicle.